Question

During PCR, which primer anneals to DNA with its 3' end towards the center of the sequence to be amplified?

1) Forward primer
2) Reverse primer
3) Both forward and reverse primers
4) None of the above

Answer 1

The reverse primer is the primer that anneals to DNA with its 3' end facing towards the center of the sequence to be amplified during PCR, ensuring the correct direction for DNA synthesis.

Step-by-step explanation:

During PCR, which primer anneals to DNA with its 3' end towards the center of the sequence to be amplified? The answer is 2) Reverse primer. In the PCR process, primers are short pieces of DNA that are complementary to the target sequence at each end. The forward primer anneals to the 5' to 3' strand with its 5' end towards the center, while the reverse primer anneals to the 3' to 5' strand with its 3' end towards the center.

This orientation ensures that during the annealing phase, each primer is extended in the right direction during the PCR cycles, eventually leading to the amplification of the target sequence. After the initial cycle, the primers are more likely to anneal to the newly synthesized DNA than to the original template, resulting in an exponential increase in the quantity of the desired DNA segment.

Answer 2

In PCR, it is the reverse primer that anneals to the DNA with its 3' end facing towards the center of the sequence to be amplified. The correct option for the question is 2) Reverse primer, which is necessary for the target DNA region's successful amplification.

Step-by-step explanation:

During PCR (polymerase chain reaction), the process of amplifying a specific DNA sequence involves the use of two primers: the forward primer and the reverse primer. Each primer is designed to be complementary to opposite strands of the target DNA sequence. It is the reverse primer that anneals to the DNA with its 3' end facing towards the center of the sequence to be amplified. This is because the reverse primer is complementary to the 5' to 3' sequence found on the opposing strand of DNA. After heating denatures the DNA strands, the reaction mixture is cooled to allow the primers to hydrogen bond to their complementary sequences. The Taq polymerase then extends these primers, synthesizing new strands of DNA.

Unlike natural DNA replication, which copies the entire genome, PCR targets specific regions using these sequence-specific primers. Important to understand is that in the early cycles of PCR, the resulting DNA products extend beyond the sequence of interest, but by the second cycle and onwards, the desired product, which is flanked by the primer sequences, is exponentially amplified. Therefore, the correct option for which primer anneals with its 3' end towards the center of the target sequence is 2) Reverse primer. This is critical to the successful amplification of the target DNA region in PCR.